1. Field of the Invention
The present invention relates to a variable displacement compressor suitable for use in vehicular air-conditioning system, and more particularly relates to an improvement in a variable displacement compressor of the type having a gas extracting passage providing fluid communication between a crank chamber and a suction chamber.
2. Description of the Related Art
In the related art, as a variable displacement compressor able to change its displacement, there is known one including a cylinder block provided with a plurality of bores around its center axis, a drive shaft inserted into a shaft hole of the cylinder block and supported thereby to be rotatable about its center axis, a swash plate supported by the drive shaft inside a crank chamber to be able to change an angle of inclination thereof with respect to a plane vertical to the center axis of the drive shaft, pistons coupled with the swash plate and moving reciprocally inside the bores, a housing closing off an end face of the cylinder block and having a suction chamber for a refrigerant before compression and a discharge chamber for the refrigerant after compression, a gas extracting passage providing constant fluid communication between the crank chamber and the suction chamber, a gas feed passage providing fluid communication between the crank chamber and the discharge chamber, and a displacement control valve for opening and closing the gas feed passage.
In this compressor, when the suction chamber pressure falls below a set value, the displacement control valve opens the gas feed passage in response to the pressure. When the suction chamber pressure rises above the set value, the displacement control valve closes the gas feed passage.
Therefore, when the compressor is operated at full capacity with the displacement control valve closing the gas feed passage, the refrigerant gas blowing by from the compression chambers in the bores to the crank chamber always flows through the gas extracting passage back to the suction chamber, to maintain the difference between the crank chamber pressure and the suction chamber pressure at an extremely small value and hold the swash plate at the maximum angle of inclination. When the suction chamber pressure falls below the set value in accordance with a decrease in the thermal load in the air-conditioning system, the displacement control valve is opened, to feed a high pressure refrigerant gas from the discharge chamber to the crank chamber while causing an increase in the crank chamber pressure. In other words, the difference between the crank chamber pressure and the suction chamber pressure becomes larger, and the angle of inclination of the swash plate is gradually reduced to reduce the discharge capacity of the compressor. Later, the thermal load again starts to increase due to the continuation of the low displacement operation. When the displacement control valve is closed in accordance with a rise in the suction chamber pressure above the set value, the crank chamber pressure falls because the refrigerant gas always flows passage from the crank chamber through the gas extracting passage to the suction chamber, that is, the angle of inclination of the rotating swash plate is increased. Thus, the crank chamber pressure is adjusted in accordance with the suction chamber pressure. Based on this, the angle of inclination of the swash plate is adjustably changed and the displacement of the compressor is controlled.
In the above-mentioned variable displacement compressor, while the transition from large displacement operation to low displacement operation can be achieved by positively feeding discharge refrigerant gas into the crank chamber, the refrigerant gas in the crank chamber is constantly allowed to return through the gas extracting passage to the suction chamber. Namely, a part of the refrigerant gas compressed by the compressor is used for controlling the displacement of the compressor per se. In this control system, when the sectional area of the gas extracting passage is large, the amount of the gas fed into the crank chamber at the time of transition to low displacement operation increases proportionally to the sectional area of the gas extracting passage. Thus, the amount of refrigerant gas wastefully used for transition from the large displacement operation to the low displacement operation must be increased to result in a large power loss. Therefore, in order to effectively increase the crank chamber pressure by a small amount of feed gas and to reduce the above power loss at the time of transition to the low displacement operation, the gas extracting passage needs to be formed to have a small sectional area.
If the sectional area of the gas extracting passage is made smaller, however, the sludge and other foreign matter contained in the refrigerant gas is liable to clog the gas extracting passage and results in the function as a gas extracting passage being completely lost.
Further, in the above variable displacement compressor, the gas extracting passage extending between the suction chamber and the crank chamber is normally formed to pass through the cylinder block. Further, due to the demands for reducing the weight of the compressor, aluminum alloys have recently been used as the material for cylinder blocks and pistons, but when drilling a hole of a small diameter as a gas extracting passage in a cylinder block made of an aluminum alloy, there is also a problem in that the processing accuracy and productivity are reduced due to the attachment of the chips to the drill during the drilling operation.